(1) Field of the Invention P The present invention relates generally to acoustic arrays, and more particularly to a composite hydrophone array assembly having a hydrophone channel response that can be shaded along its length.
(2) Description of the Prior Art
Traditionally, ceramic hydrophones are used in towed arrays for the purpose of measuring pressure fields in a fluid. Discrete (i.e., separated) hydrophone elements are grouped together to form what is commonly known as a hydrophone group or channel as it will be referred to hereinafter. The hydrophone elements are connected together electrically in parallel or series or a combination of both. The hydrophone channel is connected to the input of a preamplifier. Each preamplifier is connected to the input of a telemetry channel. The array output is comprised of a number of distinct hydrophones whose signals are time delayed and summed so that the array is steered in many directions to search the volume of the liquid for incident acoustic waves.
In the traditional array construction with ceramic hydrophones, a hydrophone channel is centered within a foam cylinder for positioning within a hose. The associated electronics are mounted in a similar fashion and the hose is filled with a nonconductive fluid such as silicone oil. Strength members needed to sustain the drag loads developed during towing are either molded into the hose wall or exist as cords within the oil and foam inner volume of the confines of the hose.
The array is spatially shaded at a beamformer by weighting numerically, usually in software, the outputs of each hydrophone channel. The purpose of shading is to reduce the level of the side lobes of the array response thereby increasing the ability of the array to discriminate between signals of interest. The array response can be discussed either as a function of bearing relative to its longitudinal axis or as a function of wavenumber as will be used hereinafter to describe the hydrophone channel or array response.
In the traditional hydrophone array, the hydrophone channel is not shaded because of the difficulty involved. The amplitude and/or phase of the discrete hydrophone elements have to be individually adjusted to achieve a particular variation in sensitivity as a function of length. This is a very costly process and is usually not undertaken. A problem that results from the traditional uniformly weighted hydrophone channel design is that the first side lobe of the hydrophone is only 13 dB down from the main lobe with successive side lobes rolling off at 6 dB per octave. These hydrophone channel side lobe levels can admit unwanted noise due to flow or structural vibration. A second problem with the traditional hydrophone channel design is that there exist hydrophone channel grating lobes set by the physical spacing of the hydrophone elements. These hydrophone channel grating lobes admit unwanted noise from the sources previously described for the hydrophone channel side lobes.
Skinner, U.S. Pat. No. 4,672,592, provides a cylindrical transducer for use as a low frequency hydrophone using a thin-walled cylinder made up of transducer material. Electrodes are positioned on the external and internal surfaces of the transducer material. One electrode of the transducer is deposited in a pattern wherein the electrode coverage varies as a function of transducer axial length. Capacitance variations between the inner and outer surfaces are used to detect acoustic signals; accordingly, great care must be take when manufacturing the transducer to insure that close tolerances are maintained between the two electrodes.
Pearce, U.S. Pat. No. 5,357,486, provides a piezoelectric film strip transducer wherein the film is wrapped around a flexible, inert mandrel a number of times. Standoff collars or bosses are provided on each end of the piezoelectric film strip section to separate an outer coating from the film. Variations in pressure in the region between the film strip and the outer coating cause the film strip to flex in tension thereby generating a voltage.